The present invention relates-to pipe for use in sewers, storm drains, penstocks, culverts and other low head applications, and more particularly to a hydraulically efficient pipe which is also adapted for use with an interior inert protective lining without the need for additional securement means.
Corrugated metal pipe of both the annular and helical type is currently widely used for culverts and other similar pipelines due to its relatively low cost and light weight which facilitates handling. However, its corrugated construction and susceptibility to corrosion have heretofore greatly restricted its use in storm drains and prevented altogether the use of corrugated metal pipe in sanitary applications such as sewer installations. Consequently, industry has been forced to turn to the considerably heavier and more expensive concrete pipe for such pipelines.
The cylindrical wall of corrugated pipe is sinusoidal in cross-section giving the pipe a rough interior surface which has a direct adverse effect on fluid flow therethrough. This effect is clearly illustrated by Manning's Equation: ##EQU1## wherein "n" represents the coefficient of roughness, V represents the velocity of flow in feet per second, the hydraulic radius and S the slope or grade. Smooth concrete pipe has a coefficient of roughness of about 0.013 as opposed to about 0.027 for corrugated pipe having 3.times.1.0 inch corrugations and 0.024 for pipe having corrugations of 2.66.times.0.5 inch. Due to this inverse effect on the velocity of fluid flow, the use of corrugated pipe requires a larger diameter for a given flow than pipe with a relatively smooth interior such as concrete pipe and the larger the pipe (generally necessitating larger corrugations or heavier wall thickness for additional structural strength), the greater the difference. It would therefore be desirable to provide a type of pipe having the cost and weight advantages of corrugated steel pipe, but with improved hydraulic efficiency.
In addition to having poor fluid flow characteristics, metal corrugated pipe is highly susceptible to corrosion from the material flowing therethrough. Accordingly, the steel from which such pipe is made is almost always galvanized. In some cases an inert protective coating is also applied to the interior of the pipe in an effort to provide additional protection against corrosion. However, such coatings have also proved ineffective in many installations as the turbulent fluid flow through such pipe caused by its rough interior surface causes debris such as rocks and the like to be dragged or rolled along the sinusoidal corrugations abrading these protective coatings resulting in erosion and corrosion and pipe damage. In an effort to prevent such erosion and corrosion, the interior of corrugated steel pipe has been lined with concrete in the hopes that a thicker lining would be more abrasion resistant and thereby resist deterioration and corrosion. In addition, the smooth interior surface presented by the concrete would improve the hydraulic efficiency of the pipe. However, there is no suitable means for anchoring the concrete to the interior wall of corrugated pipe and pieces of the concrete lining inevitably begin to fall therefrom. This destroys the improved flow characteristics of the pipe and when combined with the continual abrading action occurring therein, quickly destroys the protective concrete layer as well. If a concrete liner or other suitable material could be adequately anchored to the pipe wall, the resulting combination would not only be highly suited for use in storm drains but would be markedly improved over the conventional concrete pipe due to reduced costs and weight.
In addition to accelerating corrosion by causing abrasion of the metal's protective coatings, a corrugated surface also causes a build-up or collection of foreign material on the corrugations. Such a build-up also leads to corrosion and, in fact, prevents such pipe from being used in sewers or sanitary applications where bacterial build-up can occur. In such uses it is necessary to employ pipe having a relatively smooth bore, not solely from a hydraulic standpoint, but to facilitate cleaning of the interior of the pipe and prevent the breeding of bacteria. Here again, industry has had to turn to concrete pipe despite the fact that concrete is highly susceptible to attack by sulfuric acid which is created by the hydrogen sulfide generated in sewer lines. For sewer installations as well as storm drains, it would be highly desirable to be able to utilize the less expensive and lighter steel pipe with an inert interior protective lining. While concrete would not be preferable for such applications, as above indicated, an inert lining such as one constructed of polymer materials such as polyethylene or PVC, which would resist the attack of sulfuric acid as well as other forms of corrosion, would be ideal. In addition, such a liner should be sufficiently thick to provide protection against abrasion. Because the conventional interior linings of corrugated pipe are so susceptible to abrasion and corrosion and a thicker and more abrasive resistant inert lining such as one constructed of concrete or an inert polymer material cannot be effectively anchored to the corrugated pipe walls, corrugated steel pipe has heretofore been unacceptable for use in sanitary applications such as sewer drains.
Just as corrugated metal pipe suffers from interior difficulties, a problem also exists in adequately protecting its external surface against corrosion. Pipelines are generally laid beneath the ground and again steel is quite vulnerable to its environment. While plastic and other protective coatings can be applied to the pipe's exterior, a single holiday in the coating opens the door to damaging corrosion. The generally rough manner in which pipe lengths are handled in the yard, during loading and unloading and on the job, substantially reduces the effectiveness of these coatings and consequently the advantages of corrugated steel pipe as well.
In view of the shortcomings of corrugated metal pipe and concrete pipe, it would be highly desirable to provide a pipe having structural and cost characteristics similar to those of corrugated metal pipe, but with improved flow characteristics and capable of being readily rendered corrosive resistant, both interiorly and exteriorly, and, for use in sanitary applications, easily provided with a continuously smooth interior liner. Such pipe could be more economically employed in those applications in which corrugated steel pipe is presently being used and also would be ideally suited for sewers, storm drains and other areas heretofore outside the field of use of metal corrugated pipe.